The present invention relates to the processing of hot steel strip in rolling mills. More particularly, the invention relates to a method of equalizing the temperature of hot steel strip, and to an apparatus for carrying out this method.
Modern technology makes increasingly stringent requirements with respect to the uniformity of quality and characteristics of rolled steel strip, particularly wide steel strip. This uniformity is to apply both with reference to the width of the strip and with reference to the length of the strip. To meet these requirements it is necessary to take corrective action as early as possible in the manufacturing process. Thus, one step which is taken in the making of the steel is the increased use of continuous coating. Other steps can be taken in the rolling mill in which the billets are first passed through a preliminary mill where they undergo a first deformation by approximately 60-80%. The thus coarsely rolled hot steel strip (this entire disclosure is, of course, directed to hot rolling) is then advanced via a roller conveyor to the final rolling mill. The steel strip enters into the final rolling mill relatively slowly which means that all those portions of the hot steel strip which remain on the roller conveyor awaiting their turn to enter the final rolling mill, tend to cool. This cooling is extremely non-uniform over the length and width of the steel strip, a phenomenon which is of course not acceptable in light of the earlier-mentioned requirements.
It has therefore been proposed to counteract the non-uniform cooling in lengthwise direction of the strip by speeding up the final rolling process in the final rolling mill, so that the dwell time of any particular portion of strip on the roller conveyor intermediate the preliminary and final rolling mills is reduced. This has, in fact, been largely effective. Not effective, however, have been the various attempts to counteract differential cooling over the width of the strip, at least not to the extent of being both operationally and economically acceptable. One proposal suggests covering the entire roller conveyor to prevent heat loss. Such covering is, however, subjected to high thermal stresses, particularly in the center area, so that its construction is necessarily expensive and the costs involved in maintaining it fully operational are high. This makes the proposal economically not feasible.
A further proposal invisions heating the marginal portions of the steel strip on the roller conveyor by means of special heaters, in order to raise their temperature to the same temperature as the center portion of the steel strip. This, also, is economically not tenable because the world energy situation requires that energy be saved as much as possible in manufacturing processes. In terms of the treatment of hot steel strip on a roller conveyor connecting the preliminary rolling mill with the final rolling mill this means an effort to reduce heat losses, and particularly the differential heat losses from the steel strip as much as possible, since this goal--if it can be achieved--will ultimately permit a reduction of temperatures in the preliminary rolling mill.
A method and an apparatus have been proposed in German published application No. 1,452,102 which permit a control of the temperature fluctuation of hot steel strip over the length of the strip, in the area between the preliminary and final rolling mills. A control of temperature fluctuation over the width of the strip is not possible. Moreover, this proposal uses thermally insulating and thermally absorbing materials which means that there will be high thermal stressing of the system and considerable system maintenance involved.